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J.R. Horne, N.E. Leach, W. Miller, C. Busuioc, C. Young, S. Quintero, J.P. G. Bergmanson; Orbscan II Generated Quantification of Keratoconus . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1322.
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© ARVO (1962-2015); The Authors (2016-present)
Keratoconic (KC) corneas do not usually present all associated signs, especially the mild KC, which often exhibits no detectable signs with the biomicroscope. The present study utilizes scanning slit technology of the Orbscan II instrument to uncover quantifyable differences between normal and KC corneas.
Eight subjects diagnosed with KC were compared to nine age–matched controls using an Orbscan II topographer to determine the anterior and posterior floats and the central versus peripheral corneal thickness. Central thickness for normals was defined as the thinnest location within central cornea and for KC the thinnest point within the cone. Peripheral corneal thickness was defined as the thickest peripheral pachymetry reading of the cornea for both normal and KC. Data was statistically analyzed by a paired t–test utilizing Minitab software and using a 95% confidence interval.
Differences between maximum and minimum values for anterior float (AF) measurements were significantly different (p=0.0005) between KC (avg=155.9um) and control (avg=51.2um). A significant difference (p=0.0000) in posterior float (PF) differences (max–min) was also found between the two groups (KC 253.3um vs control 76.9um). Pachymetry readings taken with the Orbscan II showed the average thinnest corneal thickness to be 415um for KC and 561um for controls. Average thickest pachymetry readings were within the published range for normals for both groups, but significantly different between the two groups (KC 645um vs control 698um, p=.0047). Average difference between thinnest and thickest pachymetry readings was 230um for KC and 137um for control.
Apical pachymetry readings for the KC group showed thinner corneas than those in the control group. Using the mean values for controls plus two standard deviations (d+2SD) as a cutoff for normal, then a difference between central and peripheral Orbscan pachymetry reading exceeding 174um may be regarded as an indication of KC. Again using the control, d+2SD (77um+29um), a difference between high and low PF measurements, exceeding 106um is a strong indicator of existing KC. Using the control, d+2SD (51.2um+37.2), an AF difference exceeding 88um would be suggestive of KC. In addition to curvature assessment, the scanning slit instrument provides three further measurements, anterior float, posterior float and central and peripheral pachymetry, which all may aid in the accurate diagnosis of KC.
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